In recent years, information technology, information communication technology, and multimedia technology have been greatly advanced. This increases demand for an increase in density and capacity of an optical information storage medium.
To cope with such demand, there have been developed various techniques, such as multilayer techniques for forming information recording layers in a multilayered manner and super resolution techniques using a reproduction layer that allows reading of a recording mark shorter than a resolution limit of an optical information storage medium reproduction apparatus.
Initially explained is one of the multilayer techniques. The following describes a double-layer technique, which is most simple among the multilayer techniques. As disclosed in Patent Literature 1, for example, a double-layer optical information storage medium has such a structure that a first information recording layer and a second information recording layer are provided in this order from a side of a reproduction-laser-striking surface of the double-layer optical information storage medium, and an intermediate layer is provided therebetween so that the first and second information recording layers are separated from each other. In this arrangement, since the first recording layer is a translucent layer that passes reproduction light therethrough, a reproduction laser entering from the reproduction-layer-incident surface can be focused onto either of the first and second information recording layers so that information can be recorded in or read from the either of the first and second information recording layers. Accordingly, with the use of the double-layer technique, it is possible to approximately double an information recording capacity of the optical information storage medium, by simple arithmetic.
For a structure of an optical information storage medium having at least 3 information recording layers, it is also possible to adopt, similarly to the above example of the double-layer technique, such a structure that a first recording layer, a second recording layer, a third recording layer, . . . , and an nth recording layer are provided in this order from a side of a reproduction-laser-incident surface of the optical information storage medium, and an intermediate layer is sandwiched between two of these information recording layers so as to separate them from one another.
Further, the above optical information storage media do not employ any super resolution techniques. Therefore, in the optical information storage media, a shortest recording mark length in each of the layers is longer than a resolution limit of an optical information storage medium reproduction apparatus.
The following explains about the super resolution techniques. The super resolution technique is a technique for read a signal having a recording mark length shorter than an optical resolution limit (hereinafter, referred to as optical diffraction limit) in a reproduction optical system. More specifically, a light spot diameter is represented by λ/NA substantially, where λ is a wavelength of light emitted from a light source and NA is a numerical aperture of an objective lens for forming a light spot.
That is, the super resolution technique is a technique for recording/reading a recording mark having a recording mark length equal to or less than this optical diffraction limit. Further, reproduction of an optical information storage medium with the use of the super resolution technique is referred to as super resolution reproduction.
Further, it has been known that a length of a resolution limit of a conventional optical information reproduction apparatus, which does not use the super resolution technique, is around λ/(4NA), which is one fourth of the light spot diameter. In the following description, this limit is simply referred to as a resolution limit. It should be noted that since the resolution limit is actually affected by elements in an optical system in addition to the theory, there might be some differences between an actual value of the resolution limit and a theoretical value obtained from the wavelength and the numerical aperture.
As described above, with the use of the super resolution technique, it is possible to record/read a recording mark having a recording mark length longer than the resolution limit. This can improve an information recording capacity of the optical information storage medium.
Patent Literature 2 proposes a super resolution optical information storage medium that uses, as the super resolution technique, a super resolution technique utilizing prepits each made up in a form of a concavity or a convexity, which super resolution technique contributes to reading of information called Super-ROM.
This super resolution optical information storage medium has not been clarified yet in detail in terms of a reproduction mechanism. However, by using Mo, W, Si, Ge, and the like material instead of Al and Au, which are conventionally used, to form a reflection layer of a read-only disk, it is possible to read a signal having a recording pit length shorter than the resolution limit, which signal cannot be read by a conventional optical system.
Further, a so-called multilayer super resolution technique, which is a technique that combines the aforementioned multilayer technique and super resolution technique, that is, a technique in which the super resolution technique is applied to each information recording layer in a multilayer optical information storage medium, is expected as a technique that allows a further increase in the information recording capacity. The multilayer super resolution technique may adopt the aforementioned double-layer technique as the multilayer technique. It is called a double-layer super resolution technique.